1. Technical Field
The invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device having an image sensing function.
2. Related Art
A liquid crystal display device controls light transmittance of liquid crystal by use of electric field, thereby displaying a picture. The liquid crystal display device includes a liquid crystal display panel where liquid crystal cells are arranged in a matrix shape, and drive circuits for driving the liquid crystal display panel.
The liquid crystal display panel includes a thin film transistor array substrate and a color filter array substrate which are disposed to be opposite to each other. Spacers are located between the two substrates to keep a uniform cell gap. A liquid crystal is filled in the cell gap.
The thin film transistor array substrate includes gate lines and data lines. A thin film transistor (“TFT”) is formed as a switching device at each intersection of the gate lines and the data lines. Pixel electrodes are formed by the liquid crystal cell and connected to the thin film transistors. An alignment film spreads over the pixel electrodes. The gate lines and the data lines receive signals from the drive circuits through each pad part. The thin film transistor supplies the pixel electrode with a pixel voltage signal supplied to the data line in response to a scan signal supplied to the gate line.
The color filter array substrate includes color filters formed by the liquid crystal cell and a black matrix for dividing the color filters and reflecting an external light. A common electrode commonly supplies a reference voltage to the liquid crystal cells and an alignment film spreads over the common electrode.
The liquid crystal display panel is completed by injecting and sealing the liquid crystal after making the thin film transistor array substrate and the color filter array substrate and bonding them together. The liquid crystal display device operates to display an image. The liquid crystal display device may not have a function that senses and displays an external image which is realized as a content picture such as an external document or image. A separate device such as an image sensing device may be needed to sense the image.
FIG. 1 is a diagram illustrating a related art image sensing device 5. The image sensing device 5 includes a photo TFT 40, a storage capacitor 80 connected to the photo TFT 40, a switch TFT 6 located in the opposite direction to the photo TFT 40 with the storage capacitor 80 disposed therebetween.
The photo TFT 40 includes a gate electrode 8 formed on a substrate 42; an active layer 14 overlapping the gate electrode 8 with a gate insulating film 44 interposed therebetween; a drive source electrode 60 electrically connected to the active layer 14; and a drive drain electrode 62 opposite to the drive source electrode 60. The active layer 14 is formed to overlap the drive source electrode 60 and the drive drain electrode 62, and further includes a channel part between the drive source electrode 60 and the drive drain electrode 62. An ohmic contact layer 48 is further formed on the active layer 14 for being in ohmic contact with the drive source electrode 60 and the drive drain electrode 62. The photo TFT 40 operates to sense the light which is incident by a designated image such as a document or a human finger print.
The storage capacitor 80 includes a storage lower electrode 72 connected to the gate electrode 8 of the photo TFT 40, an insulating film 44 and a storage upper electrode 74 formed to overlap the storage lower electrode 72 and connected to the drive drain electrode 62 of the photo TFT 40. The storage capacitor 80 stores the electric charge generated by a photo current. The photo current generates in the photo TFT 40.
The switch TFT 6 includes a gate electrode 8′ formed on a substrate 42; a source electrode 10′ connected to the storage upper electrode 74; a drain electrode 12′ opposite to the source electrode 10′; and an active layer 14′ which overlaps the gate electrode 8′ and forms a channel between the source electrode 10′ and the drain electrode 12′. The active layer 14′ is formed to overlap with the source electrode 10′ and the drain electrode 12′ and further includes a channel part between the source electrode 10′ and the drain electrode 12′. An ohmic contact layer 48′ is further formed on an active layer 14′ for being in ohmic contact with the source electrode 10′ and the drain electrode 12′. This switch TFT6 is protected from incidence light by light-protection layer 41.
The driving of the image sensing device 5 is explained. A driving voltage, e.g., about 10V, is applied to the drive source electrode 60 of the photo TFT 40, and a reverse bias voltage, e.g., about −5V, is applied to the gate electrode 8. A light is sensed at the active layer 14. There is generated a photo current path which runs from the drive source electrode 60 to the drive drain electrode 62 through the channel in accordance with the sensed light intensity. The photo current path runs from the drive drain electrode 62 to the storage upper electrode 74. The storage lower electrode 72 is connected to the gate electrode 8 of the photo TFT 40, and the electric charge is charged in the storage capacitor 80 by the photo current. In this way, the electric charge in the storage capacitor 80 is transmitted to the switch TFT 6, and the image is sensed by the photo TFT 40, can be read by a read-out IC.
The separate image sensing device includes a photo TFT, a storage capacitor connected to the photo TFT, and a switch TFT located in the opposite direction to the photo TFT with a storage capacitor therebetween.
As noted above, the liquid crystal display device and the image sensing device are separately constructed and operate to perform its own functions, respectively. Accordingly, there is a need of a liquid crystal display device having an image sensing function.